A Senate inquiry has concluded that a Russian fake-news campaign targeted "no single group... more than African-Americans."
It says Russian operatives used social media to deter black people from voting and planted subtly racist content to incite conflict between ethnicities.
The report comes two years into the Senate Intelligence Committee's investigation into the 2016 election.
Social media firms say they are trying to prevent it from happening again.
Thousands of Twitter, Facebook, Instagram and You Tube accounts created by the St. Petersburg-based Internet Research Agency (IRA) were aimed at harming Hillary Clinton's campaign and supporting Donald Trump, the committee concludes.
More than 66% of Facebook adverts posted by the Russian troll farm contained a term related to race.
African-American community voters were discouraged from voting, and from supporting Hillary Clinton.
Posts with the titles "Our Votes Don't Matter," "Don't Vote for Hillary Clinton" and "A Vote for Jill Stein is Not a Wasted Vote" were specifically targeted at black voters, the report says.
How it worked
An Oxford study cited in the Senate report captures several pictures of Facebook and Instagram posts which it says were created by the Russian operatives.
Oxford's Computational Propaganda Research Project includes pictures of posts which Russian operatives posted on Facebook and Instagram.
In these posts, operatives were told to avoid racial slurs but still post racist content, the report says, in order to evade the attention of social media monitors and not have their ads taken down.
IRA posts like this one, which pictured the confederate flag and included issues connected with the political argument about gun ownership, were meant to create conflict on social media, the report says.
The Senate intelligence report says posts were aimed at making Americans suspicious of one another, and of the US government.
Russian ads aimed at African-Americans mimicked social justice movements that were already active in the black community, and tried to create a negative response from other US ethnic groups, the report says.
The Black Matters Facebook page, created by Russian operatives, was meant to imitate the Black Lives Matters movement, which campaigned against racism.
One of the Black Matters posts, pictured above, advertised a rally for Eric Garner, who was put by police in a chokehold before he died in custody.
Daniel Pantaleo, the policeman involved, lost his job but was not criminally charged.
Garner's dying words "I can't breathe" became a rallying cry for protests against police brutality.
Other IRA ads and social media did not contain any political content at all. They were meant to go viral and gain followers.
This post by IRA account @blackstagram_ _ used a picture of a prominent social media influencer, Janee Allure. It received 111,102 likes with the hashtags, #blackexcellence and #blackgirlmagic.
Another image posted on Instagram featured a young girl in a rainbow dress with hashtags that mentioned community pride and Black History Month. It was shared 168,998 times.
Facebook users engaged 11.2 million times with a top-performing page, "Blacktivist", which was created by the Russian agency.
Almost 100% of its YouTube content involved videos of violence perpetrated against African-American men, says the report.
Alleged police brutality against African-Americans featured on Blacktivist video and Instagram posts as well as news stories with racial undertones, like the NFL kneeling protests.
How social media has changed
Facebook says it has doubled the number of people working on safety and security, and just last month met several government agencies including the FBI, as well as academics, to prevent Russian influence in the 2020 elections.
The social media site says it takes down millions of fake accounts a day, and has improved its response results across all of its teams.
"We have stepped up our efforts to build strong defences on multiple fronts...We have also invested in technology and people to block and remove fake accounts; find and remove co-ordinated manipulation campaigns; and bring unprecedented transparency to political advertising." Facebook said.
Google said: "Videos on YouTube are viewable by anyone. Users can create videos intended for certain audiences, but there is no way to target by race".
Twitter said that it received no information from the US government before the 2016 election about a state-sponsored operation.
Since "discovering the issue," a spokeswoman from Twitter said it was working with law enforcement and the FBI as well as the Department of Homeland Security, and had conducted a public investigation into the events of 2016.